Increase the number of entries in the stack trace in test-valgrind

The default seems to hide a lot of detail when the malloc call is very
deep inside a library and not in dunst code itself.

Makefile

@@ -80,6 +80,7 @@ test-valgrind: test/test
 			--leak-check=full \
 			--show-leak-kinds=definite \
 			--errors-for-leak-kinds=definite \
+			--num-callers=40 \
 			--error-exitcode=123 \
 			./test